1. Field of the Invention
This invention relates, generally, to stability of power systems. More specifically, it relates to small-signal stability analysis techniques and criteria based on impedance measurements of power electronic-based components. The real-time stability analysis technique is capable to assess the small signal stability of the power systems while they are operating (in real-time) and to monitor the systems' stability status online.
2. Brief Description of the Prior Art
Utilizing power electronic-based components (PECs) in power systems is of growing importance because these devices can considerably enhance the characteristics of the power systems such as power quality, voltage regulation, and power factor. Consequently, as the world anticipates a sustainable energy future, large scale integration of variable power generation as well as PECs into the grid is on the rise. Nevertheless, PECs may have a substantial effect on the stability of power electronic-based distribution systems (PEDS), particularly in a large scale power system with high penetration of the PECs. Therefore, the nature of stability assessment of PEDS has significantly changed due to the capability of these devices to operate as negative impedances in the systems. Constant power loads (CPLs) can represent negative impedance characteristics, from the small-signal viewpoint, in systems with a natural destabilizing effect [1]. Generally, tendency of the PECs to behave as CPLs is the main reason for instability in the PEDS [2]. In the PEDS, small-signal stability should be addressed as well as all other types of traditional stabilities such as large-signal stability. Due to this fact, stability assessment of the PEDS is more sophisticated than stability study of the conventional power distribution systems.
Although, small-signal stability study of the PECs has been addressed in numerous research studies to date, considering stability in a system comprising several PECs is the goal of these types of studies. Generally, the stability of the systems in related literature is divided into three main categories: steady-state, small-signal and large-signal [3]. Steady-state analysis is the initial step to approach a system stability study that provides significant understanding of the system behavior during normal operation. In the conventional power system stability studies, stability from a steady-state viewpoint is utilized. Herein, steady-state stability analysis indicates all types of traditional stability analysis in the power systems area which itself may be categorized as steady-state, dynamic, and transient stability in the literature [4]. In the present disclosure, it is assumed that the system is stable in steady-state since it is a prerequisite condition for small-signal stability analysis. Therefore, different types of steady-state stability are not addressed. In addition, the present disclosure addresses the stability of small-signal systems around a desirable operating point. Generally, small-signal stability techniques are developed based on average linearized models around the equilibrium points which allow the use of various analytical tools such as Nyquist, Bode, and Root-locus plots that can facilitate the study. Almost all the small-signal stability assessment techniques for the PEDS have utilized Middlebrook's criterion [5] to ensure small-signal stability of the systems by preventing the Nyquist contour of Zsource(S)/Zload(S) from encircling the (−1+j0) point in the s-plane. This condition also may be addressed from an eigenvalue viewpoint; encirclement by the Nyquist contour is identical to an eigenvalue on the right-hand plane (RHP).
Small-signal stability of the systems can be investigated in both design and operational modes. Therefore, several criteria and techniques were previously developed in the associated research. Among all the stability criteria proposed in the studies, impedance measurement-based techniques are one of the strongest and well-developed methods which have been utilized more than other methods and with different small-signal stability criteria.
The small-signal stability assessment based on the impedance measurement technique and Nyquist criterion in the present disclosure has a significant advantage over aforementioned techniques. The main advantage is the capability of this technique to be developed for real-time applications. Although real-time small-signal stability assessment of the PEDS is an extremely significant subject, it has not currently been addressed. By increasing penetration of PECs in a large scale power grid, steady-state and large-signal stability analysis cannot ensure the overall stability of the system. Small-signal stability of a system during operation should be investigated as well to prevent instabilities in distribution and sub-transmission levels of the power systems. In the present disclosure, several dominant concepts from previously developed stability techniques are utilized and by taking advantage of the mathematics an algorithm is modified in order to facilitate implementation.
Accordingly, what is needed is a novel method for real-time small-signal stability assessment of the PEDs using impedance measurement-based techniques. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this invention how the shortcomings of the prior art could be overcome.
All referenced publications are incorporated herein by reference in their entirety. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein, is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.
The present invention may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.
In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.